Terahertz spectroscopy using silicon technology is gaining attraction for future portable and affordable material identification equipment. To do this, a broadband THz radiation source is critical. Unfortunately, the bandwidth of the prior CMOS works is limited. In a previous design, a 300 GHz signal source achieves 4.5% tuning range by changing the coupling among multiple oscillators. In another, the distributed active radiator (“DAR”) array has 3% tuning range with radiation capability. Alternative to the continuous device-tuning method, THz time-domain spectroscopy utilizing the broadband spectrum of a pico-second pulse is widely used in optics community.
CMOS circuits working in the millimeter-wave and terahertz (THz) frequency range (100 GHz-1 THz) are receiving increased attention due to promising applications in the security, biomedicine, and communication areas. More particularly, recent works have demonstrated fully-integrated image sensors working up to 1 THz, and wireless data links over 200 GHz. For these applications, a signal source should advantageously generate high radiation power to overcome large propagation loss at this frequency range. Unfortunately, it is well known that a “terahertz gap” exists, which keeps the generated terahertz power low. This is because the relevant frequency range is too high for electronics, and too low for optics. In the context of CMOS technology, such difficulty is mainly attributed to three factors. First, despite the aggressive trend to scale down CMOS, the maximum frequency of oscillation, fmax, of a CMOS transistor is still below 300 GHz, especially when device interconnects are included. This sets a theoretical limitation, beyond which no fundamental oscillation nor power amplification is possible. Second, the thinner gate oxide in the advanced technology node results in lower breakdown voltage. This severely reduces the output power of the device, which is strongly correlated to the voltage swing. Third, the passive metal structures fabricated in CMOS have high loss, especially with the presence of the lossy silicon substrate. The challenge lies in the thin metal layers and the thick, lossy silicon substrate. Because of these drawbacks in CMOS, high-power terahertz generation is more commonly demonstrated in III-V compound semiconductors. For example, using InP high-electron mobility transistors (“HEMTs”), a 650-GHz power amplifier module with 3 mW output power was reported. In another previous work, 4.2 mW output power was demonstrated with a 600 GHz GaAs diode frequency tripler when cooled to 120 K (1.8 mW at room temperature).
In addition to the signal power level, another challenge in CMOS THz sources is the output frequency bandwidth. A broad bandwidth is especially important for material identification using THz spectroscopy. For example, prior research shows that many types of hazardous gas (e.g., methylchloride) and warfare chemical agents (e.g., sarin) exhibit vibrational resonance between 200 GHz and 300 GHz. To obtain such spectrum, a broadband radiation source is required.
To overcome the cutoff frequency limitation in CMOS, previous techniques utilize device nonlinearity and harmonic generation. Signal sources based on such principles can be further divided into two categories: (i) frequency multipliers; and (ii) harmonic oscillators. Frequency multipliers normally have both high output power and bandwidth. For example, in a previous work, a 180 GHz active doubler achieved 0 dBm output power and 11.1%-3-dB bandwidth. In other work, such performance metrics were achieved using a traveling-wave doubler at 275 GHz are −6.6 dBm and 7.8%. In a 480-GHz passive doubler from another work, the measured output power and frequency range were larger than −6.3 dBm and 4.2% (limited by testing equipment). However, these multipliers need a large-power and wide-tuning-range fundamental signal source to drive, which is another challenge. In comparison, the second nonlinear circuit category, i.e., the harmonic oscillator, has the advantage of being self-sustainable. The reported output power is competitive to that of the frequency doubler, especially with multi-cell power combining. For example, a 482-GHz triple-push oscillator achieved −7.9 dBm power. In another example, a coupled oscillator achieved −1.2 dBm. Normally, there is significant power loss in the process of radiation. Nevertheless, in another example, a 16-element 280 GHz distributed active radiator achieved a radiated power of −7.2 dBm and an EIRP (effective isotropic radiated power) of 9.4 dBm. Utilizing a pair of triple-push oscillators and a differential ring antenna, a high radiated power of −4.1 dBm at 288 GHz was reported. Despite such progress, large frequency tuning in harmonic oscillators remains very challenging. This is mostly due to the lossy MOS varactors used in the resonance tank. In a previous work, a variable-coupling solution effectively reduced such loss, and achieved a tuning range of 4.5%. Although producing the highest tuning range reported in prior CMOS THz oscillators, it remains insufficient for THz spectroscopy.